The present embodiments relate to a receiver assembly of a magnetic resonance (MR) system for generating MR recordings of an examination object.
MR systems include a plurality of reception channels, each including a plurality of components connected in series and through which received MR signals (e.g., reception signal, RX signal) pass. For each channel, these components include a coil acting as antenna, a preamplifier, possibly with frequency conversion, a receiver including a first analog portion (e.g., analog RX) for preparing and digitizing the signal, and a second digital portion (e.g., digital RX) for decimation filtering.
As a result of the high signal dynamic response of the received magnetic resonance signals, particularly in the case of magnetic field strengths above 1.5 Tesla, switching of the reception path distributed gain (e.g., RX gain) is provided in one or more steps (e.g., gain step, 6 dB) in order to be able to process the high dynamic response of the MR signals. In the case of two gain stages (e.g., low gain, high gain), the reception path is operated in the high gain operating state with more gain and a lower noise figure (NF) of the RX system for a certain portion of the k-space of the Fourier-transformed MR signals to be sampled and operated in the low gain operating state with less gain and slightly higher noise figure for the central region of the k-space. This switching of the reception path distributed gain is brought about by switchable damping members or switchable amplifiers (e.g., switchable gain amplifiers, SGAs) within the analog RX system prior to sampling by an analog/digital converter (ADC). Since the switching takes place within a k-space, the transmission magnitudes and phases of the respective reception paths, which are dependent on the gain settings, are to be tuned precisely to one another (e.g., calibrated), because otherwise a gain and/or phase jump is produced in the k-space. This leads to image artifacts after the Fourier transform into the image space.
Calibrating the RX system is complicated and often brought about by a function incorporated in the system. A common test transmission signal (TTX) is distributed over all components to be calibrated. After the distribution thereof over the various reception paths to be calibrated in relation to one another, these TTXs are to have the same or at least known amplitudes and phase relationships. After distributing and recording the TTX signal, the amplitudes and phases of the MR signals may be recorded relative to one another and/or relative to the set high-gain level or low-gain level, and calibration factors may be derived relative to an arbitrarily selected channel. The calibration factors eliminate the phase and amplitude effect of the gain switching on the RX signal.
Such a calibration may take place in reception channels installed in the MR system and use a relatively high time outlay. The calibration procedure is to be carried out on the MR system after each replacement of a component of the reception chain or of an entire reception chain of one or more channels.